Metallurgical slag

ABSTRACT

A method of making a material for use as a steel-making slag comprises mixing sufficient lime and/or magnesia containing materials with combustion ash until the so-formed composition has a basicity ratio appropriate for use in steel making.

BACKGROUND OF THE INVENTION

This invention relates to slag, specifically to that used in thesteel-making industry.

A slag is present on the top of the melt in a metallurgical furnace orvessel. The main function of the slag is to combine with unwantedimpurities from the melt so that they can be separated from the melt. Italso tends to prevent the melt from picking up gases from the furnaceatmosphere.

It is known that the so-called basicity ratio of the slag is importantin steel-making. The basicity ratio is defined as the ratio of basicoxides to acidic oxides.

One measure of the basicity ratio (B) is defined thus:

$B = \frac{{\%\mspace{14mu}{CaO}} + {\%\mspace{14mu}{MgO}}}{{\%\mspace{14mu}{SiO}_{2}} + {\%\mspace{14mu}{Al}_{2}O_{3}}}$

Other, sometimes more useful, measures of the basicity ratio have beenproposed (see Turkdogan E. T. The Fundamentals of Steelmaking; TheInstitute of Materials (1996)). Such measures may be expressed thus:

$B_{LF} = \frac{{\%\mspace{14mu}{CaO}} + {1.4\%\mspace{14mu}{MgO}}}{{\%\mspace{14mu}{SiO}_{2}} + {0.6\%\mspace{14mu}{Al}_{2}O_{3}}}$or $V = \frac{\%\mspace{14mu}{CaO}}{\%\mspace{14mu}{SiO}_{2}}$

Virgin slag compositions are slag compositions which are brought intocontact with the melt at the commencement of the steel making process.The values for basicity ratios, as defined above, are typically above 2for the so-called “virgin” slag compositions.

Care must be taken when using and referring to the basicity ratio asthere are many ways of defining the basicity (see, for example,Sommerville, I. D. and Yindong Yang; The AusIMM Proceedings, pp. 71,Vol. 36 No. 1; 2001).

Furnaces used in the steel industry, for example Basic Oxygen Furnaces(BOFs), Electric Arc Furnaces (EAFs) and Ladle Furnaces (LFs) are metalvessels which have a refractory lining to hold the molten metal. Therefractory materials are chosen to withstand the extreme temperatures towhich they are subjected. It is important for reasons of economy andefficiency that damage to the refractory materials is minimised. Damageto such refractory materials is controlled by, inter alia, the basicityand by MgO saturation levels.

Many industries seek to gain extra benefit out of production processesby reclaiming waste materials. Such reclamation may take the form ofrecycling unused or partially used materials or the reclamation of sayenergy in the form of heat by combusting waste materials. One suchindustry is the paper production industry where the by-product, wastepaper pulp or sludge, is often burnt. The product of the so-combustedwaste is known as combustion ash.

Combustion ash from the paper production industry is typically a finepowder substantially composed of calcium oxide, silicon dioxide,aluminium oxide, magnesium oxide and iron oxide. Small amounts of othermetallic oxides and chlorides are also usually present. Because of thephysical composition and state of combustion ash it has heretofore hadlimited economic use and is typically land-filled. In many parts ofEurope, at least, a charge is levied for the disposal and, say, landfilling of industrial waste.

It is an object of the present invention to provide a cheap source ofvirgin slag, whilst providing an economic sink for combustion ash,thereby reducing land-fill and producing a further income stream. It isa further object of the invention to provide a so-called “artificial”slag made at least partially from waste materials. It is a yet furtherand more specific object of the invention to provide an artificial slagfrom waste materials which causes no, little or not excessive damage tothe refractory materials used in steel-making processes.

SUMMARY OF THE INVENTION

Accordingly, a first aspect of the invention provides a method of makinga material for use as a steel-making slag, the method comprising mixingsufficient lime and/or magnesia containing materials with combustion ashuntil the so-formed composition has a basicity ratio appropriate for usein steel making.

A further aspect of the invention provides a material for use as a slagin the steel-making industry, comprising a mixture of combustion ash andlime and/or magnesia containing materials and having a basicity ratioappropriate to the steel to be made and/or steel making process to beutilised.

Preferably, the ratio of add-mixed lime to combustion ash is from 1:2 to2:1, calculated as parts by mass, but may be more or less depending onthe composition of the combustion ash. The ratio of add-mixed magnesiato combustion ash is preferably no more than 1:4, more preferably nomore than 1:5.

The basicity ratio (V) of the product may be advantageously adjusted toa range of from 2.0 to 9.0, depending upon the metallurgicalrequirements of the process in which the slag is intended to be used, bythe addition of appropriate amounts of lime and/or magnesia containingmaterials.

The added or add-mixed lime and magnesia are preferably in either orboth powder or granular form, most preferably each with a nominaldiameter of less than 10 mm. The lime and magnesia may be separatecomponents and/or may be in the form of dolomitic lime which may have alime to magnesia ratio of about 3:1 and/or other materials rich in limeand/or magnesia.

It will be appreciated by those of skill in the art that the term‘lime’, as used herein, typically refers to burnt lime (CaO). However,in special and rare situations, CaCO₃ may be used in addition, or as analternative to, CaO.

A yet further aspect of the invention provides a method of generatingartificial slag, the method comprising treating combustion ash toprovide a material with a basicity ratio appropriate for use in steelmaking.

Preferably, the combustion ash is treated by adding substances thereto,for example lime and/or magnesia, or removing components thereof, forexample silica and/or alumina.

A still further aspect of the invention provides the use of combustionash as at least a part component of artificial slag for use in steelmaking.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In order that the invention may be more fully understood it will now bedescribed with reference to the accompanying Examples.

Table 1, below, provides details of a typical composition of combustionash. The range shown is typical of combustion ashes although some mayvary outside of the ranges shown. All combustion ashes are considered tobe suitable for use in the current invention.

As can be seen, Table 1 shows that the specific example of typicalcombustion ash has basicity ratio values which are too low to use it asa virgin slag.

TABLE 1 Typical Composition of Combustion Ash COMPONENT Range (w/w %)SiO₂ 22-34 Al₂O₃ 15-22 Fe₂O₃ 0.7-1.3 CaO 37-47 MgO 4-6

EXAMPLE 1

To 100 parts by mass of the above-specified combustion ash were addedand mixed 50 parts by mass of lime. The components were mixed to providea substantially homogeneous mixture. The resultant mixture had basicityratios, B of 2.1 B_(LF) of 2.6 and V of 3.4. The mixture was found to besuitable as an artificial virgin slag material in the steel makingindustry.

EXAMPLE 2

To 100 parts by mass of the above-specified combustion ash were addedand mixed 200 parts by mass of lime and 20 parts by mass of magnesia.The components were mixed to provide a substantially homogeneousmixture. The resultant mixture had basicity ratios B of 5.8, B_(LF) of7.1 and a basicity ratio V of 8.4 by calculation. The mixture was foundto be suitable as an artificial virgin slag material in the steel makingindustry.

EXAMPLE 3

To 100 parts by mass of the above-specified combustion ash were addedand mixed 40 parts by mass of lime and 40 parts by mass of dolomiticlime (25% by mass magnesia, 75% by mass lime). The components were mixedto provide a substantially homogeneous mixture. The resultant mixturehad basicity ratios B of 2.8, B_(LF) of 3.4 and a basicity ratio V of4.1. The mixture was found to be suitable as an artificial virgin slagmaterial in the steel making industry.

The so-formed virgin artificial slag may be placed on top of or into themelt by means of a conveyor or the like, thereby drawing off theimpurities from the melt. Alternatively, and due to the small particlesize of the combustion ash, the so-formed virgin artificial slag may beentrained in, say, a gas stream and injected, pumped or otherwise forcedinto the melt below its' surface, so that it will remove impurities asit percolates or passes upwards.

The so-formed virgin artificial slag is useful in EAFs, BOFs and LFs orany vessel containing liquid steel and further can aid elementalphosphorus and sulphur removal from the steel (as P₂O₅ and CaSrespectively) in, say, EAFs. Further, due to the presence of MgO in theartificial slag, the tendency for MgO to dissolve from the refractorymaterials used in the liners of such furnaces is reduced, therebyminimising damage to those refractory materials.

Whilst the above-identified Examples have indicated that the combustionash and other substances are mixed prior to addition to the melt, it isalso within the scope of this invention to add the combustion ash andlime and/or magnesia separately to the melt. Thus, lime, say, may beadded to the melt and combustion ash added as a distinct component, thesum of the two parts forming a slag with a basicity ratio appropriatefor use in steel making. One component may be forced into the melt belowits surface and the other placed on top of the melt, both may be placedon top of the melt, or both may be forced into the melt below itssurface.

It will be understood by the skilled man that the sum of lime and/ormagnesia additions will be specific to a particular steel makingapplication and even to a particular steel making plant, due to, interalia, the production of oxides during the steel making process whichalso go to form part of the slag. Thus, the initial basicity ratio willbe adjusted and selected to accommodate the impurities produced in thefurnace, the quality of steel required and the type of refractorymaterial used to line the furnace.

1. A method of use of an ash derived from the combustion of waste paperpulp or sludge from paper production industry in steel making to removeunwanted impurities from molten metal, the method comprising providingthe ash derived from the combustion of waste paper pulp or sludge as atleast a partial component of an artificial virgin slag for use in steelmaking and thereby removing unwanted impurities, wherein furthercomponents of the slag comprise lime and/or magnesia.
 2. A method ofmaking a steel-making slag, the method comprising adding the ash derivedfrom the combustion of waste paper pulp or sludge from paper productionindustry and lime- and/or magnesia-containing materials into asteel-making furnace or other vessel containing liquid steel, wherein atleast some lime-and/or magnesia-containing materials are added to ensurethat the steel-making slag has a basicity ratio (V) appropriate for usein steel making.